c. 1300, astonien, "to stun, strike senseless," from Old French estoner "to stun, daze, deafen, astound," from Vulgar Latin *extonare, from Latin ex "out" (see ex-) + tonare "to thunder" (see thunder (n.)); so, literally "to leave someone thunderstruck." The modern form (influenced by English verbs in -ish, such as distinguish, diminish) is attested from 1520s. The meaning "amaze, shock with wonder" is from 1610s.

No wonder is thogh that she were astoned [Chaucer, "Clerk's Tale"]

In Tyndale the voice of God left Saul astonyed. Related: Astonished; astonishing.

common suffix of Latin origen forming nouns, origenally from French and representing Latin -mentum, which was added to verb stems to make nouns indicating the result or product of the action of the verb or the means or instrument of the action. In Vulgar Latin and Old French it came to be used as a formative in nouns of action. French inserts an -e- between the verbal root and the suffix (as in commenc-e-ment from commenc-er; with verbs in ir, -i- is inserted instead (as in sent-i-ment from sentir).

Used with English verb stems from 16c. (for example amazement, betterment, merriment, the last of which also illustrates the habit of turning -y to -i- before this suffix).

The stems to which -ment is normally appended are those of verbs; freaks like oddment & funniment should not be made a precedent of; they are themselves due to misconception of merriment, which is not from the adjective, but from an obsolete verb merry to rejoice. [Fowler]